1. Field of the Invention
The present invention relates to a substrate on which semiconductor elements can be mounted and that is made of an aluminum-silicon carbide composite material. It also relates to a method for manufacturing the substrate, a package for semiconductor elements that uses the substrate, and a semiconductor device that uses the substrate.
2. Description of the Prior Art
In recent years, as well as a mounting there has been an accelerated response of semiconductor elements degree of integration. This rapid growth has prompted focus on the effects of heat generated by the elements. As a result, a heat-dissipating substrate on which such semiconductor elements can be mounted and having excellent thermal conductivity has been required in order to efficiently dissipate the heat generated by the semiconductor elements to the outside.
Further, the heat-dissipating substrate has been required to reduce thermal strain caused at a combination interface between the semiconductor elements and peripheral members in a semiconductor package that contains the elements. Therefore, a thermal expansion coefficient of the heat-dissipating substrate that is equivalent to that of the semiconductor element and that of the peripheral member combined with the substrate is required. For example, the thermal expansion coefficients of silicon (Si) and gallium arsenide (GaAs) that are each a semiconductor element are 4.2 and 6.5, respectively, in a unit of [10−6/° C.]. Concerning a ceramic package, the thermal expansion coefficient of alumina (Al2O3) that is a general-purpose peripheral member thereof is 6.5 in a unit of [10−6/° C.]. Further, concerning a plastic package, for example, the thermal expansion coefficient of plastics is 12 through 17, which is relatively large, in a unit of [10−6/° C.]. A motherboard on which these are mounted is also plastic. Peripheral members, such as those mentioned above, have various levels of thermal expansion coefficients, and to correspond to these, the heat-dissipating substrate has employed a material whose thermal expansion coefficient is close to those of such peripheral members.
Further, recently, a minute chip size package of a wafer level size has appeared in response to a rapid rise in the wiring density of LSI for microprocessors. Accordingly, fine wires on a motherboard are being made with accelerating speed. A compact, light package component that can also be installed in, for example, a portable device is greatly desired as a component to be mounted on such a motherboard.
Tungsten (W), molybdenum (Mo), copper (Cu), and these composite materials (Cu—W or Cu—Mo) have been used conventionally as materials for the heat-dissipating substrate, as disclosed by, for example, Japanese Published Unexamined Patent Application No. S52-59572 or Published Unexamined Patent Application No. H6-13494. However, these materials are expensive and heavy in weight. Especially in a plastic package, since a motherboard on which the package is mounted is made of plastics small in rigidity, deformation will occur so as to cause difficulty in using the device if such a substrate is mounted on the motherboard. Similar to a ball grid array package (BGA package) described later, semiconductor equipment is recently connected to the motherboard by means of solder balls. Therefore, for example, if these materials are used for the heat-dissipating substrate, the solder balls will easily be damaged so as to be destroyed or deformed by the weight of the materials. A component for a package that is light in weight, that is excellent in matching a thermal expansion coefficient with peripheral members, and that is low cost is therefore in strong demand.
Concerning the package weight reduction, the same applies to the ceramic package. However, the plastic package that has undergone a considerable reduction in size is in much higher demand for its weight reduction than the ceramic package. Conventionally, in the plastic package that has less reliability than the ceramic package, the degree of integration of semiconductor elements is small, and, accordingly, the amount of dissipated heat is also small. Therefore, a quad flat package (QFP) shown on the left side of FIG. 1 has chiefly been used. However, recently, the degree of integration of semiconductor elements in the plastic package has also rapidly increased, and, as a result, the number of terminals that can correspond to it cannot be maintained. For this reason, a new packaging structure is being developed in succession.
Its circumstances will be typically shown by FIG. 1. The left object in the figure is of the aforementioned QFP type. A semiconductor element 3 is packaged with a wire lead 2 and a lead pin 4, and the whole thereof is enclosed and sealed by sealing resin 6. This is used in a case where an actual capacity is less than at most 1 W, and the number of terminals is within 250. If the actual capacity exceeds 3 W, and the number of terminals rises up to 1000, a package having a ball grid array (BGA) structure is used like those next to the left object in FIG. 1. This structure needs a heat-dissipating substrate 1, to which the semiconductor element 3 is joined by die bonding. Like that of the QFP type, the element 3 is packaged with a wire lead, but a solder ball 5 is used as a terminal instead of a pin, in order to increase the packaging density. In the upper middle object in FIG. 1, the heat-dissipating substrate 1 is a simple plate, in which a plastic substrate 7 is shaped like a frame for supporting it. In the lower middle object in FIG. 1, the heat-dissipating substrate 1 is shaped like a lid integrated with a frame part, to which the balls 5 are bonded with a tape 12. Recently, the number of terminals has exceeded 1000 as the actual capacity becomes larger, and thus the packaging structure has been changing to those shown at the rightmost side in FIG. 1. In this structure, the semiconductor element is packaged according to a method that uses a flip chip (FC) 11, and the number of ball grids on the undersurface of the plastic substrate also has greatly increased. In the upper rightmost object in FIG. 1, the heat-dissipating substrate 1 is a simple plate, and a frame-shaped metallic stiffener 10 serving to support the substrate 1 is disposed between the heat-dissipating substrate 1 and a plastic substrate 9 and is bonded to the two substrates with adhesive layers 8. In the lower rightmost object in FIG. 1, the heat-dissipating substrate 1 is of the lid type, and the balls 5 are bonded to the substrate 1 by means of a tape 8.
A weight reduction in the package and a change in its structure will advance rapidly in the future. In correlation therewith, a member to be packaged, such as a heat-dissipating substrate, is required to be light in weight and superior in thermal conductivity and be capable of easily coping with shape diversification or shape complexity. For example, its size is being made even more compact, and its shape is becoming very diversified in relation to the combination with peripheral members. For this reason, members that have high dimensional accuracy are required to be supplied at low cost and, in addition, the accuracy thereof must be maintained when practically used. In other words, members that are thinner in shape or are more complex in shape will be demanded in the future. In most cases, a substrate has been conventionally manufactured by connecting or uniting a plurality of flat plates together. However, in the future, it seems that most substrates will be constructed such that concavities/convexities with various patterns are formed on a part of the substrate and are integrated with each other, depending on the positional relationship with other components of a package. Further, it seems that the outer periphery of the main surface will be required to have a complex shape. These demands are strongly made especially onto small sized to medium-sized semiconductor devices used for general-purpose electronic equipment. However, demands are also gradually being made onto semiconductor devices larger than medium-sized devices, exduding plastic packages.
FIG. 2 shows one example thereof Various forms of the heat-dissipating substrate are shown at the left of the figure. In FIG. 2, reference numerals are omitted except for the uppermost object. Each reference numeral designates the same component as that shown in FIG. 1. The uppermost substrate is a conventional one shaped like a simple plate. However in order to obtain a heat-dissipating area, for example, the substrate is required to have a shape in which a fin is formed in the heat-dissipating area, as shown in the middle object next to the simple-plate-shaped substrate. Concerning the left objects from the top to the bottom in FIG. 2, lid type substrates provided with variously shaped cavity parts are needed depending on the position or size of a semiconductor element to be mounted. If a fin is given to each of these substrates, they can be formed as shown at the middle of the figure. Packaging structures are shown at the right of the figure. In each of the packaging structures, the semiconductor element is subjected to the flip chip packaging by use of the substrate provided with the fin as shown at the middle of the figure, and the resulting package is mounted onto a motherboard by the ball grid array. In the uppermost structure, a frame stiffener must be interposed between the heat-dissipating substrate and the plastic substrate. Therefore, considerable time is consumed to package it, causing heat distortion to occur easily between the members because the connection parts increase. Accordingly, since the entire package is warped or deformed when packaged or operated, a reduction in reliability easily occurs. On the other hand, if a lid type of integrated substrate structure is employed as shown in the second to fifth objects on the right of the figure, such a problem will be easily solved. The conventional heat-dissipating substrate is at a disadvantage in that the substrate becomes expensive because of a rise in manufacturing costs if the substrate is complicated and integrated as mentioned above.
On the other hand, in order to meet the demand of weight reduction of the heat-dissipating substrate, the use of metal chiefly composed of, for example, ceramics or aluminum (Al) that is excellent in thermal conductivity has been considered. Concerning ceramics, aluminum nitride ceramics (AIN) and silicon nitride ceramic (Si3N4) are introduced by, for example, Published Unexamined Patent Application No. H9-175867. However, the thermal expansion coefficient of these materials is about 3 through 4 in a unit of [10−6/° C.], which is close to that of a silicon semiconductor element but is very low relative to that of plastics. Therefore, the application thereof especially to a plastic package is difficult. Ceramics have the drawback of a rise in material cost, in that deformation or damage occurs when manufactured, and in that machining is difficult. These bring about a rise in cost.
On the other hand, the drawback of metal chiefly composed of the latter, i.e., Al, is that the thermal expansion coefficient of pure Al is about 24 in a unit of [10−6/° C.], which is very high relative to not only that of a semiconductor element but also to that of plastics, and in that deformation or damage occurs easily because of its softness. Due to these circumstances, recently, to a composite (Al—SiC) of silicon carbide (SiC), which is excellent in thermal conductivity and light in weight, and Al has drawn interest. The Al—SiC is manufactured according to an infiltration method in which Al is infiltrated into a porous body of SiC as described in Published Unexamined Patent Application No. H2-243729; according to a sintering method in which Al powder and SiC powder are mixed at a desired composition ratio, and thereafter a resulting molded piece is sintered at a temperature above the melting point of Al as described in Published Unexamined Patent Application No. H10-335538 or Published Unexamined Patent Application No. H9-157773, according to a casting method in which molten Al is injected into a mold and is cast while mixing SiC powder therewith as described in Patent Application (kohyo) H1-501489. According to a dissolution method in which a plate is formed from molten Al, from which SiC particles have been dispersed, by cooling it continuously. The thermal expansion coefficient of Al is about 24, and that of SiC is about 4 in a unit of [10−6/° C.]. In this material, a thermal expansion coefficient can be controlled within a wide range by changing the amount of both components, and making it light weight. Therefore, this is one of the suitable materials for a heat-dissipating substrate used for not only a plastic package but also various packages. For this reason, practical use as a component for a package is rapidly advancing.
On the other hand, in a semiconductor device, the degree of integration of a semiconductor element is rapidly increasing as mentioned above. Accordingly, a package in which the element is contained and its component is required to have high thermal conductivity that a conventional device does not have, and is required to cause its thermal expansion coefficient to be closer to that of a peripheral member has been demanded. In addition, its size is being made more compact, and its shape is being more diversified in relation to the combination with the peripheral member. It is important for these components to be inexpensively supplied.
When viewed from the above-mentioned viewpoint, the composite material Al—SiC has several drawbacks as follows. When manufactured according to the infiltration method, according to the sintering method, according to the casting method, or according to the dissolution method mentioned above, considerable time is consumed for the finish machining of a material after compositional processing is carried out, and, disadvantageously, tools wear out in a short time because SiC particles are hard. Therefore, if the component is simple in shape, this problem is relatively easy to solve, but if the shape of a package is further thinned, further complicated, and further diversified, this problem is exacerbated. Additionally, if the component is required to have higher dimensional accuracy in correlation with peripheral members, the provision of a material capable of simplifying its finish machining will be in strong demand. It is therefore an object of the present invention to inexpensively provide an Al—SiC semiconductor substrate that is superior in thermal performance and that is capable of stably maintaining high dimensional accuracy when practically used while coping with the current of rapid diversification of a practical shape. It is another object thereof to provide a package in which a semiconductor element using the substrate can be installed and provide a semiconductor device using the substrate.